The sentence, "Søstrene mine er på kino i dag." means that they are at the cinema. 'er på kino' implies that they are there now. It would be enough to say that without 'i dag', if you just wanted to say what they are doing or where they are. Adding 'i dag' (at least in the area where I live) implies that they will be there for a while, or are having a day out at the cinema, or something. Maybe they are watching more than one film, or at a film festival?
Going to the cinema would be 'går på kino' or if they are not walking, it might be 'drar til kino'.